SH mode guided waves are elastic waves that have displacement of particles that is parallel to the surface of a structure, are guided by the structural boundary, and are propagated at a large distance. The SH mode guided waves have advantages in that they are insensitive to a fluid that may exist on an inside or outside surface of the structure and a possibility of mode conversion is low when interacting with discontinuities and thus they allow a simple echo structure that can be easily interpreted during long-range ultrasonic inspection. SH0 mode and T(0,1) mode guided waves are particularly useful in a flat plate or a plate-shaped structure having curvature and a cylindrical structure, respectively, since they have non-dispersive characteristics that their speeds of propagation are not varied according to wave frequencies. In related arts, these waves are generated or detected using a piezoelectric array transducer and two kinds of electromagnetic acoustic transducers (EMATs), i.e., a periodically polarized magnet (PPM) EMAT and a magnetostrictive transducer. The magnetostrictive transducer has a simpler structure than the piezoelectric array transducer and the PPM EMAT.
For transmission of the SH mode guided waves, the magnetostrictive transducer relies on deformation of ferromagnetic material due to overlapping of a bias static magnetic field and a dynamic magnetic field that are perpendicular to each other in a portion that is near a lower portion of the surface of the ferromagnetic material. The bias field is provided by a permanent magnet or an electromagnet to the ferromagnetic material, and the dynamic field is provided by coils through which an alternating current (AC) pulse in an RF band flows, so-called radio frequency (RF) coils to the ferromagnetic material. Due to skin effect of the dynamic magnetic field, a wave source is limited to the vicinity of the surface of the ferromagnetic material. A guided wave mode propagating along the structure mainly depends on the characteristic of the wave source and the thickness of the structure. During reception of the SH mode guided waves, the RF coils are used to detect a change of magnetic flux that is generated in the material due to the waves. When an object to be tested is formed of a ferromagnetic material, the object itself can be used as an element of a transducer so that the SH mode guided waves can be generated in the ferromagnetic material and detected without direct contact between other two elements (coils and magnet) and the object. Such non-contact magnetostrictive transducers allow high-temperature inspection. Low frequency SH-guided-waves have been transmitted and received by an elongated-spiral coil transducer, while high frequency SH-guided-waves have been transmitted and received by a meanderline coil transducer or a multi-spiral coil transducer. These magnetostrictive transducers include permanent magnets or electromagnets that generate a static magnetic field that is parallel to a direction of a leg portion of each RF coil and thus is perpendicular to a dynamic magnetic field.
In a non-ferromagnetic object, the SH mode guided waves may be transmitted and received by using contact magnetostrictive transducers each including a magnetostrictive strip (or magnetostrictive patch) that is temporarily or permanently adhered to the surface of the non-ferromagnetic object. These contact magnetostrictive transducers have also been applied to the ferromagnetic material for more efficient transmission and reception of the SH mode guided waves. In related arts, residual magnetization in the lengthwise direction of the magnetostrictive strip that is obtained by moving a U-shaped permanent magnet along the magnetostrictive strip that is adhered to a structure using a sticky material such as epoxy, has been used as a bias static magnetic field. In the contact magnetostrictive transducers, low frequency (generally less 200 kHz) SH mode guided waves propagating in the widthwise direction of the magnetostrictive strip can be efficiently transmitted or received. Thus, the contact magnetostrictive transducers have been widely used in long-range ultrasonic inspection of a large-sized structure. However, these conventional magnetostrictive strip guided-wave transducers have the following drawbacks. First, when the magnetostrictive strip is detached from the object to be tested or when epoxy firmly adhered to the detached strip is removed, the magnetostrictive strip may be easily damaged and thus, it is difficult to reuse the magnetostrictive strip repeatedly. Second, in case of a strip adhered to an object having large curvature such as a pipe having a small diameter or a non-ferromagnetic pipe such as an aluminum pipe, it is difficult to obtain uniform residual magnetization. Third, a strong dynamic magnetic field may cause a irreversible change of residual magnetization and thus, currents that flow through the RF coils during transmission of the SH-guided waves need to be limited to a certain range. Since the impedance of the RF coils is proportional to frequency, limitation of currents that flow through coils driven at relatively lower frequency becomes severe. Furthermore, the limitation is not easily quantified. This means that considerable cautions are needed for correct use of the magnetostrictive strip guided-wave transducers. Fourth, it is difficult to control intensity of residual magnetization. This implies that for construction of a transducer presenting a linear response with respect to a change of the dynamic magnetic field, the use of an optimized bias magnetic field is rarely possible.